After Glados crushes your dreams, head into the test chamber. Portal over to the far side and push the button. Set the light bridge up so both the cubes land safely on the ground and push the button again. Take the laser cubes over to the laser and use them to take out the turret. Then position then through all four laser nodes and into the receiver at the end.
There are three cubes in this room, use portals to get all three down on the floor. For the third one you will need to do some quick change flinging. To do so, put a portal above the faith plates, and then one on the floor near them. Jump through the portal and when you fly back out of it, change the portal above the faith plates to where you want to fling from. When you fall back through the one on the ground, you will fling out like a normal fling.
Using normal portals and the quick change flinging, get you and the cubes up to the laser, and zig zag it across the map, going through all the nodes. Before redirecting the last one into the receiver, be sure to destroy the turrets by the door with the laser. After that, you simply need to quick change fling over to the exit.
Use the two laser cubes to put the three lasers into one portal at the three angles in the picture. Then put the other portal in front of the three receivers by the exit door.
It's the last test chamber, and it's actually pretty easy. Just get the bridge up by the button, press it and walk across the bridge to get the cube- wait, what happened?
Wheatley came through, and is busting you out of here! Follow Wheatley out of the test chamber as he explains his plans.
You will be trapped in a room with turrets. After knocking them over, use the white wall to portal out through a small gap in the wall. After walking a bit more, you will find another turret. Simply put a portal underneath it and keep going.
Glados is now trying to crush you with the walls. Just keep running until you get into the elevator.